Silica hydrogel is the essential precursor to silica xerogel which is a versatile material for industrial purposes and has a number of industrial applications, for example as an adsorbent, desiccant, catalyst base, toothpaste component, beer clarification, filler and flatting agent in coating compositions. The hydrogel is obtained from a hydrosol formed by acidification of a silica containing solution. The hydrogel is purified by removal of electrolyte and dried to provide a gel with the appropriate pore volume and surface area. After or during washing the hydrogel may be subjected to ageing steps to obtain a range of silica gels with different structures and properties.
The commercially used silica containing material is an alkali metal silicate which may be sodium or potassium silicate, with the former being preferred. The acidification step is usually performed by the addition of a mineral acid, for example sulphuric acid or phosphoric acid with the former being used commercially. In existing technology the hydrogel is prepared by acidifying the alkali metal silicate and allowing this to set to a hydrogel, normally on a moving conveyor approximately from 10 to 25 m in length. This provides sufficient residence time for the gel to gain the required mechanical strength to enable it to be broken into manageable lumps (0.5 to 5 mm in diameter), and also to prevent the broken pieces congealing into large gel masses in the washing vessel.
The hydrogel is broken into pieces by various methods for example allowing it to fall through a grid or passing it through rotating cutter blades. The hydrogel pieces are then washed in water of suitable purity to remove the electrolyte impurities. Normally this washing step is performed in tanks of considerable size, containing 1 to 10 tonnes of hydrogel, and may occupy several hours to obtain efficient leaching of the impurities. The hydrogel is usually washed in hot water, at 60.degree. C. to 80.degree. C., for a period of 6 to 12 hours depending on the residual electrolyte level required in the final product. Alternatively cold water can be used for washing but this increases the washing time by a factor of at least three. Thus the efficient washing of hydrogel on commercial scales requires considerable equipment, energy input for the wash water and a relatively long washing cycle period to reduce the electrolyte content to the required level. The variation in particle size can lead to inconsistent washing and non-uniform pH zones in the gel lump, which in turn, at elevated temperatures can subject the hydrogel to ageing before the washing stage is complete.
For products where a low surface area and high pore volume are required the washed hydrogel is subjected to structural modification by extending this hydrothermally ageing at elevated temperatures, about 90.degree. C., and about neutral pH for long periods of time, i.e. about 6 to 10 hours. After ageing the hydrogel is dried and if required milled to the size for the intended application.